1. Field of the Invention
The present invention relates generally to a laser anneal apparatus for use in a process of manufacturing a semiconductor integrated circuit, more particularly to a laser anneal apparatus suitable for a process of manufacturing a thin film transistor for use in a flat panel type display device.
2. Description of the Related Art
In a flat panel type display device such as an active matrix type liquid crystal display device or an organic EL display device, a large number of thin film transistors (hereinafter abbreviated as TFT) are formed on an insulating substrate made of glass or plastic in order to individually drive pixels. Since an amorphous silicon (a-Si) film can be relatively easily formed at a low forming temperature by a gas phase process, and has a superior productivity, the film is broadly used as a semiconductor thin film for forming source, drain, and channel areas of the TFT.
However, the amorphous silicon thin film has a drawback that the film is inferior to a polycrystal silicon (poly-Si) film in physical properties such as conductivity (mobility of a-Si is lower than that of poly-Si by a factor of two digits or more). Therefore, it has been requested that the method of forming the source, drain, and channel areas of the TFT in the polycrystal silicon thin film be established in order to increase an operation speed of the TFT.
In the present situation, for example, an excimer laser annealing process (hereinafter referred to as the ELA process) using excimer laser is adopted as a method of forming the polycrystal silicon thin film (Flat Panel Display 1999/Nikkei Micro Device Separate Volume, pp. 132 to 139 (Nikkei BP Inc., 1998)). This annealing process can be performed in a temperature range (e.g., from room temperature to about 500° C.) in which a versatile glass substrate is usable. In the ELA process, for example, after depositing the amorphous silicon thin film into a predetermined thickness (e.g., 50 nm) on the substrate, this amorphous silicon thin film is irradiated with krypton fluoride (KrF) excimer laser light (wavelength of 248 nm), xenon chloride (XeCl) excimer laser light (wavelength of 308 nm) or the like to locally melt and recrystallize the amorphous silicon thin film, and the film is changed into the polycrystal silicon thin film.
When an average intensity (fluence) of the laser light is changed, the excimer laser annealing is applicable to various processes including the crystallization of the amorphous silicon thin film. For example, when the intensity of the laser light is set to a region only having a heating function, the annealing can be used in an impurity activation step which is required for manufacturing the TFT. When the intensity of the laser light is excessively raised, a rapid temperature rise is caused. Therefore, the annealing can be utilized in removing films during the manufacturing of the TFT. It is to be noted that utilization of these phenomena is applicable to not only the TFT but also a general semiconductor manufacturing process.
However, in a case where the TFT is constituted of the polycrystal silicon thin film in order to increase the operation speed in a flat panel type display device such as a liquid crystal display device or an organic EL display device, when there is a fluctuation in the number or distribution of crystal grain boundaries included in the channel area of each TFT, a large fluctuation is generated in a threshold voltage (Vth) between the TFTs. This becomes a factor to deteriorate operation characteristics of the whole display device and lower an image quality. Therefore, it has been demanded that the crystal grain boundary in each channel area be homogenized as much as possible or a crystal grain diameter be set to be larger than a size of the channel area and a position of the crystal grain be controlled to thereby remove the crystal grain boundary from each channel area.
The present inventors investigate a laser annealing process for forming a silicon thin film which has a large crystal grain diameter. According to the process, when an optical device called “phase shifter” is inserted into an optical path irradiated with the laser light to thereby adjust a secondary intensity distribution of the laser light on the amorphous silicon thin film, crystal grains are grown into large sizes. Here, the phase shifter is an optical device in which a fine planar pattern such as line and space is constituted of stepped portions including recessed and protruding portions in a transparent quartz substrate. In the phase shifter, a phase difference is made in a part of the laser light which passes through the shifter, and the secondary intensity distribution of the laser light is produced by diffraction of the laser light and interference between the laser light having different phases. Therefore, when the secondary intensity distribution of the laser light is adjusted, a temperature distribution is generated on a substrate to be treated. Accordingly, silicon single crystals having large grain diameters of about two to seven microns can be formed while positions of the crystals are controlled.
The following problems have been found in developing such laser annealing process.
A first problem accompanies generation of high-order diffracted light. To stably form a crystallized region having a required large grain diameter, an intensity distribution pattern in a micro region of a sub-micron level is very important as to the laser light with which the amorphous silicon thin film is irradiated. The phase shifter having fine recessed/protruding portions is used in order to form such intensity distribution pattern with a good efficiency. However, such phase shifter generates the high-order diffracted light in accordance with pitches of the lines and spaces or the recessed and protruding portions, and there is generated a large quantity of light that does not enter the substrate to be treated. For example, when the phase shifter having a pitch d [μm] of the line and space is perpendicularly irradiated with the laser light having a wavelength λ [μm], order-n diffracted light is generated in addition to rectilinearly traveling light (order-0 light). An angle θn formed by the order-n diffracted light and the order-0 light is given by the following equation:θn=sin−1(nλ/d),wherein 0≦θn≦90°: n=0, ±1, ±2, . . .
That is, when the order (n) increases, the angle (θn) formed by the order-n diffracted light and the order-0 diffracted light increases. Therefore, the high-order diffracted light having the increased angle formed with the order-0 diffracted light enters an inner wall of the apparatus or another optical system (optics), and causes an unintended problem.
Even in the optical device other than the phase shifter, for example, a photo mask having a metal thin film provided with an opening which passes light, unintended high-order diffracted light is sometimes generated depending on a shape of the opening. In general, as a countermeasure against the light which does not enter the substrate to be treated, a method is adopted in which a path from the optical device to an image-forming optical system is disposed in a sealed box, and the high-order diffracted light is confined in the box. However, in this method, the high-order diffracted light is simply absorbed by the inside of the apparatus or a wall surface of the box. Such light is not utilized in the crystallization at all. Moreover, adverse effects are caused by the absorption of the light. For example, inner components are deteriorated (e.g., photo or thermal deterioration), and a gas or the like is generated owing to the temperature rise and tissue change caused by the absorption of the light.
It is to be noted that in an exposure device for use in photolithography, an aperture is disposed in an intermediate portion of the image-forming optical system in order to remove the high-order diffracted light. However, in an activation or crystallization process of the semiconductor thin film, since the laser light has high energy, and the high-order diffracted light has intense energy, such countermeasure has its limitation.
A second problem accompanies “non-emitted shot” in the laser anneal apparatus. It is to be noted that in the following description, the “non-emitted shot” includes both of a case where any laser pulse is not generated and a case where the laser intensity is low or an emission time is short.
In a case where the laser pulse is intermittently emitted, when there is temporarily a defect in laser oscillation for a certain cause, an annealing defect is generated unless information indicating the defect is fed back to the laser anneal apparatus. For example, when any laser pulse is not generated or the pulse intensity is lower than a target value, an insufficiently annealed area is generated, and yields of final products (e.g., display device) are deteriorated. Specifically, since a partial area is not crystallized at all, or the crystallization or the activation is insufficient in the crystallization process performed by the laser annealing, targeted transistor characteristics or characteristic uniformity cannot be obtained, and a pixel defect or color unevenness is generated.
Usually, excimer laser has a high annealing capability accompanying ultraviolet light, but an oscillation characteristic is unstable. Specifically, although a trigger signal is input from the outside, the laser sometimes fails to oscillate. Even if the laser oscillates, oscillation intensity is excessively weak (e.g., half or less of a target value). In the excimer laser, the pulse oscillation is caused by high-voltage discharge in a gas, and the above-described instability is attributable to characteristics of a current element called a thyratron for use in the high-voltage discharge. Therefore, it is difficult to eliminate the above-described instability at the present time. Since such problem appears as a fluctuation of the crystal grain diameter in the laser annealing process, the problem obstructs mass production.
It is to be noted that excimer laser which does not use any thyratron has been developed, but a burden on a power circuit is large, and there is a problem of reliability drop or cost rise.
In the above-described document, there is described a method of judging whether or not a laser light shot is the non-emitted shot. According to the method, an optical component which splits a part of a luminous flux is disposed between a laser source and the substrate to be treated, and the intensity of the split light is monitored. When it is judged that the intensity of the split light is not normal, the position is returned to a position corresponding to the shot, and the laser pulse is emitted again. However, such method has the following problem. Firstly, since the laser light is split, the intensity of the laser light with which the substrate to be treated is irradiated is lowered. Secondly, since many optical components such as the light-splitting optical component and a photo detector have to be installed in the apparatus, the apparatus is enlarged and complicated.